In fields such as geotechnics and civil engineering, it has long been known the need to characterize the behaviour of structures through analysis of their oscillations, according to an engineering practice called “modal analysis”.
Within this practice, the oscillations of structures are traditionally energized by suitable impulsive energy sources, typically impacts or small explosions.
However, it has been recently found that by using high sensitivity seismographs it is possible to avoid the use of any external source and to study the oscillations, entirely similar although very much smaller, induced by the seismic “tremor”, that is by vibrations at frequencies from 10−4 to 102 Hz which have occasionally an anthropic origin, but which have in general a natural origin (wind, waves, tides, vulcanic activity, etc.), and which are always present everywhere on the surface of the Earth.
Seismic tremor oscillations have a much smaller amplitude than the oscillations normally measured by standard seismographs and, in actual fact, in seismology the seismic tremor is considered as a “noise” that disturbs the “signal” of interest, which is composed of the waves emitted by earthquakes.
Since seismic tremor acts locally as an energizing function for the resonance frequencies of buildings and of the subsoil, recording and identifying these resonance frequencies makes it possible to measure in a completely passive, and therefore extremely economical, way the structural characteristics of buildings and of the ground beneath them.
Currently, such measurements are made by standard seismic instrumentation which, due to the fact that it has not been designed specifically for this purpose, is somewhat ineffective and prone to measurement errors. Moreover, such instrumentation is in general heavy, bulky, expensive and requires the intervention of more than one operator.